An optical semiconductor device such as a semiconductor laser used in an optical communication system or an optical information processing system has been required to have higher performances as well as to be fabricated with a low cost for public use. Accordingly, it is necessary to fabricate such an optical semiconductor device from a large size semiconductor wafer with a high yield. In order to meet these requirements, the optical semiconductor device is fabricated by a process in which crystal growth is carried out by vapor phase epitaxy such as metalorganic vapor phase epitaxy (MOVPE) by which epitaxial growth is realized in a large area with high uniformity. Such a vapor phase epitaxy enables the fabrication of a semiconductor laser of quantum well structure having characteristics such as a low threshold and high efficiency of operation or narrow spectrum operation.
In a first conventional method for fabricating an optical semiconductor device, a double-hetero (DH) structure consisting of an n-InP clad layer, an InGaAsP active layer and a p-InP clad layer is formed on an n-InP substrate. Then, an SiO.sub.2 layer is formed on the p-InP clad layer, and patterned to be stripes each having a width of 2 .mu.m. Then, the DH structure is mesa-etched, except for the area which is masked by the SiO.sub.2 layer, until the surface of the n-InP substrate is exposed. The width of the remaining active layer thus experienced the mesa-etching becomes 1.5 .mu.m, which enables the foundamental transverse mode to be kept stable. Then, p-InP and n-InP buried layers are deposited in this order on the exposed surface of the n-InP substrate to stuff the grooves formed by the mesa-etching on both sides of the remaining double-hetero structure. Finally, after removing the SiO.sub.2 layer, a p-InP layer and a p.sup.+ -InGaAs cap layer are deposited in this order to cover the fabricated surface of the n-InP substrate.
In a second conventional method for fabricating an optical semiconductor device, a DH structure consisting of a first clad layer, an active layer and a second clad layer is formed on a substrate. Then, an SiO.sub.2 layer is formed and patterned to be stripes each having a predetermined width. Then, the DH structure is mesa-etched, except for the area which is masked by the SiO.sub.2 layer, until the surface of the n-InP substrate exposes. Then, after removing the SiO.sub.2 layer, a third clad layer and a cap layer are deposited in this order to cover the fabricated surface of the n-InP substrate including the DH structure. Finally, a high resistance region is formed on both sides of the DH structure by implanting protons to confine a current flowing within the DH structure.
On the other hand, there is a great demand for an integrated optical semiconductor device such as one including a distributed feedback (DFB) semiconductor laser and a semiconductor optical modulator with narrow spectrum at high speed modulation or one including a distributed Bragg reflection (DBR) semiconductor with variable wavelength.
In a third conventional method for fabricating an optical semiconductor device, a grating is formed only within the DBR region on the surface of an n-InP substrate. Then, an n-InGaAsP guide layer, an active layer, a p-InP clad layer are deposited in this order.
Then, the p-InP clad layer and the InGaAsP active layer in all of the regions, except for the active region are removed with using an SiO.sub.2 layer as a mask. Then, an InGaAsP waveguide layer and a p-InP clad layer are grown selectively on a predetermined region. Then, the fabricated surface of the n-InP substrate, except for the areas forming the elements, are mesa-etched with using an SiO.sub.2 layer as a mask. Then, an Fe-doped high resistance InP layer and an n-InP layer are grown to be buried. Then, after removing the SiO.sub.2 layer, a p-InP layer and a p.sup.+ -InGaAs cap layer are grown. Then, grooves for insulation are formed between the laser and waveguide regions, and between the adjacent semiconductor lasers. Then, an SiO.sub.2 layer is deposited on throughout the fabricated surface of the n-InP substrate. Then, after forming openings of the SiO.sub.2 above the waveguide areas of the modulator, DBR, phase adjusting and active regions, and p-electrodes are formed to connect the waveguide areas thereof through the openings, while an n-electrode is formed on the back surface of the substrate.
According to the first to third conventional methods for fabricating an optical semiconductor device, however, there is a disadvantage in that it is difficult to control width of waveguides or active layers precisely, because the conventional methods include the steps of semiconductor etching using an SiO.sub.2 layer as a mask.
In semiconductor etching such as mesa-etching, the thickness of the semiconductor can be controlled precisely, however, the width thereof varies because of variation of the mesa structure of occurrence of side etching. Variation of width of the active layer or waveguide may affect the characteristics of the device such as the threshold current, oscillation wavelength or beampattern. Consequently, such a variation of the width thereof may cause yield reduction or outranging of designed operation.
Furthermore, in the fabrication, it is required to match the location of the active layer with that of the waveguide, so that the steps of etching and growth of buried layers become complex and have problems in uniformity and reproducibility which affect characteristics of the device and yield in fabrication.